As is well known in the art, in a display device used in a television receiver or the like, R, G, and B phosphors are arranged regularly in a CRT (cathode-ray tube). These phosphors are activated by beams from a red gun, a green gun, and a blue gun, respectively, according to red, green, and blue video signals, respectively. Thus, the phosphors emit light of different colors to provide a display of a picture.
CRTs including electron guns vary widely among manufactured products. Also, red, green, and blue phosphors do not have uniform characteristics. The cutoff point at which a phosphor begins to emit differs among the three kinds of phosphors.
The cutoff characteristic, or the luminescent characteristic, is represented in terms of the ratio of the cathode current to the cathode voltage, and is shown in FIG. 15. As can be seen from this graph, the phosphors emitting red (R), green (G), and blue (B), respectively, differ in cutoff characteristic. It is to be noted that FIG. 15 merely shows one example. It is not always the case that the cutoff points lie in the order of red, green, and blue in order of lowering cathode voltage. The cutoff characteristic is quite nonuniform among CRTs.
During manufacture of CRTs, adjustments are needed to bring the cutoff points into agreement. In particular, a level shift circuit, a gain control circuit, and other circuits are included in a signal processing/amplifier network for each of the three primary colors. These level shift circuit and gain control circuit are adjusted to bring the cutoff characteristics of all the phosphors into agreement.
An automatic cutoff adjustment method of automatically adjusting the cutoff points has been proposed to eliminate such cutoff adjustment steps and gain control circuits. FIG. 16 is a block diagram of main portions of a television receiver utilizing the conventional automatic cutoff adjustment. After supplied video signals are demodulated into red, green, and blue signals by demodulator circuits (not shown), these three signals are fed to switch circuits 1R, 1G, and 1B, respectively. In the switch circuits 1R, 1G, and 1B, reference pulses RP.sub.R, RP.sub.G, and RP.sub.B are added to different portions within the vertical retrace interval of each signal as shown in FIG. 17.
The signals to which the reference pulses RP.sub.R, RP.sub.G, and RP.sub.B are added are supplied to driver circuits 3R, 3G, and 3B, respectively, via the level shift circuits 2R, 2G, and 2B, respectively. Of course, a picture is displayed according to the camera signal components of the signals supplied to the driver circuits 3R, 3G, and 3B. The internal configurations of driver circuits 3G and 3B are not shown, since they are the same as the internal configuration of the driver circuit 3R. The CRT is indicated by numeral 4.
The reference pulses RP.sub.R, RP.sub.G, and RP.sub.B added within the vertical retrace interval are now discussed. Similarly to the camera signal portions, these reference pulses are amplified by their respective transistors Q.sub.1, giving rise to cathode currents R.sub.IK, G.sub.IK, and B.sub.IK for the CRT. These cathode currents R.sub.IK, G.sub.IK, and B.sub.IK are detected by their respective transistors P.sub.1 and supplied to a resistor R.sub.1 via switch circuits 5R, 5G, and 5B, respectively. These switch circuits 5R, 5G, and 5B are so controlled that their contacts are closed only during periods corresponding to the reference pulses RP.sub.R, RP.sub.G, and RP.sub.B. As a result, three voltage pulses as shown in FIG. 18 are obtained by the resistor R.sub.1, the pulses corresponding to the cathode currents produced in response to the reference pulses RP.sub.R, RP.sub.G, and RP.sub.B for the three primary colors.
These voltage pulses are applied to a clamping circuit 6 via a clamping capacitor C.sub.1. This capacitor C.sub.1 is electrically charged and discharged such that the clamped portion of the voltage produced from the clamping pulse-generating portion 6a is V.sub.1 (FIG. 18). The clamped pulse voltages are successively distributed to their respective comparators 8R, 8G, and 8B for the three primary colors, respectively, by a switch circuit 7. A voltage higher than the voltage V.sub.1 by .DELTA.V.sub.1 is applied as a reference voltage to the other terminals of the comparators 8R, 8G, and 8B. The voltage pulses whose DC levels are clamped at V.sub.1 are compared with this voltage (V.sub.1 +.DELTA.V.sub.1).
The comparators 8R, 8G, and 8B are so controlled that they compare their respective one input signals with the reference voltage only while the voltage pulses are being supplied. Each comparator can take the form a differential amplifier producing an output signal containing errors included in the voltage pulses and the reference voltage. The output signals .DELTA.V.sub.1 (=.DELTA.V.sub.1 ') from the comparators 8R, 8G, and 8B are supplied to sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B, respectively. The voltages held by the sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B are supplied as control voltages to the level shift circuits 2R, 2G, and 2B, respectively.
That is, the level shift circuits 2R, 2G, and 2B are controlled by the voltages held by the sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B in such a way that the black levels of the red, green, and blue signals become equal to the heights of the reference pulses RP.sub.R, RP.sub.G, and RP.sub.B, respectively. In this way, automatic cutoff adjustment is accomplished.
The conventional automatic cutoff adjustment circuit constructed as described above has the following problems. A portion 10 surrounded by the dot-and-dash line in FIG. 16 is normally composed of one IC. The sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B must be attached to the outside of the IC. This increases the number of components. Also, three extra connection pins of the IC are used, thus increasing the cost. Furthermore, the efficiency of the manufacturing steps is deteriorated.
In addition, an electrical current flows through each of the pins to which the sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B are connected only during the period of the reference pulse (1H) within one field. Consequently, the impedance is very high. However, if the substrate ages and its impedance drops, a leakage current is produced. As a result, the operation of the sample-and-hold capacitors C.sub.2R, C.sub.2G, and C.sub.2B is no longer maintained normal.